Mariner&#39;s compass



M.IB. FIELD.

MARINERS COMPASS. APPucmon'mEu mac. 20, ms.

1,419,836. PatentedJun 13, 1922.-

i arr MICHAEL B. FIELD, OF GLASGOW, SCOTLAND, ASSIGNOR T0 KELVIN BOTTOIv/[LEY & BAIRD, LIMITED, 0]? GLASGOW, SCOTLAND, A CORPORATION OF GREAT BRITAIN.

MARINERS cor tress,

Specification of Letters Patent.

Patented June 13, 1922.

Application filed December 20, 1918. Serial No. 267,625.

To all whom it may concern:

Be it known that 1, MICHAEL BIRT FIELD, a subject of the King of the United Kingdom of Great Britain and Ireland, and residing at Glasgow, Scotland, have invented a certain new and useful Improvement in Mariners Compasses, ofwhich the follow ing is a specification My invention relates to improvements in or relating to so-called liquid magnetic compasses, and has special reference to liquid compasses of the navigational type having cards say of 5"diameter and upwards, although my invention is also applicable to liquid compasses of smaller sizes.

With the liquids at present used in liquid compasses the card may be said to be over-damped, that is to say, the fluid'drag on the cardis so great that the movement tends to become sluggish; to overcome this tendency, it has been customary to use mag netic needles oi": far higher magnetic moment than is advisable on account'of other considerations. With such powerful magnetic needles the moving system is heavy and requires considerable flotation by means of an air-filled dome, or the equivalent, in order to reduce the friction on the pivot. Heaviness ot the moving system and large magnetic moment are disadvantageous.

The consequence of the fluid drag upon the moving system is noticeable when the containing bowl is rotated; for example, it the ship steer round in a complete circle, the compass bowl is rotated once. Such rotation sets up movement of theliquid, which, reacting on the card, produces a temporary deviation.

Objects of my invention are, firstly, with a given size of card and bowl, to employ a smaller magnetic moment than has h1therto been practicable, other properties of the compass being the same, or, conversely, with the same magnetic moment as is used at present, to employ a larger sizedcard without impairing other characteristics of performance of the compass, or, ceteris paribus, to provide a compass having a shorter period.

Secondly, with a compass card of a given size. to reduce the effect of turning motion of the bowl in terms of deviation of the card, or conversely, to permit the use of a larger card, other things being equal.

following description.

In order to reduce the period, without increasing the magnetic moment, it has been proposed to reduce the moment of inertia" of the moving system. I have found, however, that if the diameter of the card and magnetic momentof the moving system remain the same, the maximum reduction oi moment of inertia of a well designed moving system permitted by any further modification of design does not result in a very great reduction of the period. I have thus been led to suspect that the rotational motion set up in the liquid by the oscillating card reacts upon the latter in such a manner that the card behaves as though it were loaded with an additional moment of inertia, besides being acted upon by a resisting torque which is at every instant proportional to the angular velocity, that is to say, what is known as a true fluid friction effeet. s

For example, I find as the result of experimental research that'if a card or disc be oscillated about its axis in the liquid besides the drag which would be produced in consequence of true fluid friction effect there is an additional drag the same-as would be produced it the card were loaded by an additional moment of inertia, such with the actual moment of inertia of the moving system I have found, therefore, that in order to arrive at a radical improvement it is necessary to decrease as far as possible by the choice of a suitable liquid the amount oi the effective additional moment of inertia. This amount is reduced when the density or the co-efticient of viscosity of the liquid, or both are decreased.

The other properties of the liquid, necessary or advisable, are that it'shall be able to withstand without freezing very low temperatures such as are met with in the Arctic regions, that it shall be able to withstand without boiling or developing an undue vapour. pressure the temperatures met with in the equatorialregions ofthe earth, and that it shall be transparent, colourless, and inert in the presence of metals, and preferably,

' oflvis c o sity of approximately .0033. in C,

S. nits and a density of about .663. 1 do notconfine myself, however, to any one liquid or to'any one series. For the par pose of the, invention the, chemical con-- stitiition. of the liquid, employed isrelaH tivelyl of'no moment; a liquid is suitable according as it possesses the essential physical properties above indicated, it being understood hat I i the product of the density and co-efiicient. of viscosity of the liquid at a temperature of from 20 to 25 C,

should not be more than say one fourth of the product of the density andco-eflicient of viscosity of a well known standard a] cohol and 1 water, mixture, the approximate density ofnvhiclr mixture, within the range of temperatures stated, is .95 and the approximate co-eflicient of viscosity of which, within the range of. temperatures stated, is .025; in C. Gr.- S. units. (See Smithsonian, Physical Tables)- The liquid which, I prefer to use is a.

; volatile saturatedhydrocarbon having the j with the liquid. contents of-the bowl, and

formula C H the product of whose density and co-eilicient of viscosity, at a temperature between about 20 C. and 25 C., is not more than about one fourth of the product of they density and co-eliicient of viscosity of the nsualalcohol and water mixture.

Certain liquids, which are otherwise suit ableon account of their low viscosity, maybe objectionable on account of being too..volatile or of exercising a destructive 'artionon the usual jointingmeans provided in a coinpass bowl. 7

p 1- provide, therefore, a chemically indestructible joint. or joints for the container forjthe liquid and sealsuch joint or joints by means of liquid;

The sealing liquid may be, for example.

glycerine,.which is inert on such iacking material as rubber, which is not miscible which is not liable to be frozen.

In constructing the moving system Ihave a1medatmak1ngthe moment of inertia as small as'ppssibl'e, and the whole system as light. as possible, and in, so. doing I have arrived; at structural improvements now to be describedwith. reference to the accom panying drawing in which Fig. 1 is a vertical section through the compass bowl, Fig. 2 a plan of the card and Fig. 3 a sectional detail view showingthe arrangement for ensuring equilibrium of pressure ontheinterior andexterior of the joints.

.l-lereto'fore, it has been usual to arrange the needles as parallel chords of a circle.

llow, I- arrange the needles 1 parallelto a diameter, but I, dispose them'as' shown so that the shorter needles are nearer to the centre of the card 2 thanthe longer needles and so that the ends of the needles, or rather the poles of the needles, lie.upon opposed V-shaped figures, the apices. of which are at or near the centre; thus, all north poles lie upon a V, theangle of which is approximately GOdegrees, while all south poles lie upon a diametrically opposed.- V. of the same angle.

' It is known tllflilCOliSlClGI'flblG adyantages accrue fromthe disposition of twoparallel needles, the north poles of which and also the south poles subtend angles of 60 degrees at the centreof the card, but I am notaware that it has been proposed to arrange a plurality of needles on each side of; a diameter in such a manner that the holes of each pair comply with this condition. The arrangement I. have adopted is not only favourable from the magnetic. point. of View,

but it further provides a smaller moment of inertia than when the needles are ar-. ranged as usual heretofore as parallel chords of a circle.-

the use of the describedarrangement of needles, and by the choice of, a suitable liquid, I am enabledto use needles of much smaller weight than has been considered practicable heretofore for a liquid compass of the same diameter of card, and hence with a smaller size, of card, say, from l downwards, no. flotation devices. need bev added. For larger sized cards, as, for ex,-

ample, for a 61; card, a certain amount of flotation is desirable Instead of employing the usual. heavy. metallic a1r-filled dome, in orderto provide.

flotation without adding materially to. the.

mass. of the moving system, I prefer to employa hollow glass floatior floats, say two small,

equivalent with exceedingly thin wallsv which may be attached to the moving system by meansof alight wire cage, 4:, and: are arranged as near to the. pivot asis practicableported by an apertured platform 6,of inica resting on an annular shelf 7 secured to the interiorverticai wall ,8 .of the. compass. bowl.

the spaces interior to and exterior to the blown glass spheres, 3 or, the.

Tlie'eentral pivot stem 5 is shown .as sup-.

vertical wall 8 and permitting transference of air bubbles from the inner chamber to the chamber 9. i

The annular chamber 9 is in turn accommodated in a chamber 10 containing a sealing fluid which does not readily volatilize, which is preferably immiscible with the liquid in which the card is poised, and which is preferably inactive on the rubber or other packing interposed between the metallic bezel ring 11 and the'glass cover 12 and between the lower ring 13 and th glass bottom 14 (it being understood that the packing rings 15 which are exposed to the action of light fluid, hexane, for example, are of some material such as lead which is not liable to be attacked by hexane or the like).

As best seen in Fig. 3, the outer wall of the annular chamber 9 is provided with apertures closed by flexible diaphragms 10, so that equlibrium of pressure on the interior and exterior of the joints 15 may be maintained.

A screw plug 16 preferably fitted with a suitable packing washer and entered into an apertured socket fixed to the wall 8 in register with a plug 17 also fitted with a packing washer and entered into an apertured socket fixed to the exterior wall of the chamber 10 permits of introduction of liquid to the bowl and replacement of liquid to compensate for leakage.

18 denotes an annulus soldered to the wall of the bowl and bearing degree markings.

The annulus 18 being below the plane of the card 2, the markings thereon are not obtrusive when the card is being observed by the steersman or otherwise read for taking ma netic directions.

(in the other hand, when the compass is used in conjunction with an azimuth mirror, the markings on the annulus 18 fall upon the line of sight of an observer looking down into the compass bowl at an angle of, say, 60 degrees to the horizontal.

The lubber line or point 19 is shown as supported by the wall 8. It will thus be understood that the wall 8 carries the whole essential mechanism of the compass, that is to say, the platform 6 and therewith the central support of the card, the lubber point, and the compass card, and all these may be inserted in place as a single unit, care being taken that the lubber line is properly orientated with regard to the axis of gimbaling.

What I claim is 1- 1. A magnetic compass comprising a card and a chambercontaining a liquid medium in which said card is poised, said medium being a volatile saturated hydrocarbon having the formula C H the product of whose density and co-efiicient of viscosity, at a temperature between about 20 C. and 25 C., is not more than about one fourth of the product of the density and co-efficient of viscosity of the usual alcohol and water mixture.

2. In combination, a magnetic compass card, and a plurality of needles carried by said card on each side of the center, said needles on each side of the card being arranged parallel to a diameter of the card with the poles of said needles located'upon imaginary opposed V-shaped figures which converge towards the centre of the card and whose apices are substantially at the centre of the card, the angle of each V-shaped figure being substantially 60, the length of the needles increasing proportionately to their distance from the centre of the card.

3. In a magnetic compass, the combination of a chamber containing a liquid, packing rings at the joints of the walls of said chamber, a card poised in said liquid, a second chamber containing a sealing liquid and surrounding said first mentioned chamber, a third chamber intermediate said first and second mentioned chambers, said third chamber communicating with the first chamber, the outer wall of said third chamber having apertures, and flexible diaphragms closing said apertures.

In testimony whereof I have signed my name to this specification in the presence of two subscribing witnesses.

M. B. FIELD.

lVitnesses ISABEL RoLLo, KATE FOTI-IERINGHAM. 

